Use libravatar over gravatar.
Deprecate .gravatar_url in favor of .avatar_url.
Restrict arguments to gravatar_url as per @abadger's comments.
Use the pyLibravatar package for .avatar_url() as per @fmarier's suggestion.
Add note to NEWS for new avatar_url method.
Use libravatar by default only if installed.  Otherwise, default to gravatar.
One more thing:  raise a good error.

...if service is explicitly given as libravatar but the module is not installed
s/ImportError/ValueError/
Merge pull request #38 from fedora-infra/feature/libravatar

Use libravatar over gravatar.